Merge pull request #856 from chriseth/sol_modifiers

Function modifiers.

22 files changed, 630 insertions, 146 deletions

diff --git a/AST.cpp b/AST.cpp
index 82667367..cc7da715 100644
--- a/AST.cpp
+++ b/AST.cpp
@@ -41,10 +41,15 @@ TypeError ASTNode::createTypeError(string const& _description) const
 	return TypeError() << errinfo_sourceLocation(getLocation()) << errinfo_comment(_description);
 }
 
+TypePointer ContractDefinition::getType(ContractDefinition const* _currentContract) const
+{
+	return make_shared<TypeType>(make_shared<ContractType>(*this), _currentContract);
+}
+
 void ContractDefinition::checkTypeRequirements()
 {
-	for (ASTPointer<InheritanceSpecifier> const& base: getBaseContracts())
-		base->checkTypeRequirements();
+	for (ASTPointer<InheritanceSpecifier> const& baseSpecifier: getBaseContracts())
+		baseSpecifier->checkTypeRequirements();
 
 	checkIllegalOverrides();
 
@@ -53,6 +58,9 @@ void ContractDefinition::checkTypeRequirements()
 		BOOST_THROW_EXCEPTION(constructor->getReturnParameterList()->createTypeError(
 								  "Non-empty \"returns\" directive for constructor."));
 
+	for (ASTPointer<ModifierDefinition> const& modifier: getFunctionModifiers())
+		modifier->checkTypeRequirements();
+
 	for (ASTPointer<FunctionDefinition> const& function: getDefinedFunctions())
 		function->checkTypeRequirements();
 
@@ -89,15 +97,22 @@ FunctionDefinition const* ContractDefinition::getConstructor() const
 
 void ContractDefinition::checkIllegalOverrides() const
 {
+	// TODO unify this at a later point. for this we need to put the constness and the access specifier
+	// into the types
 	map<string, FunctionDefinition const*> functions;
+	map<string, ModifierDefinition const*> modifiers;
 
 	// We search from derived to base, so the stored item causes the error.
 	for (ContractDefinition const* contract: getLinearizedBaseContracts())
+	{
 		for (ASTPointer<FunctionDefinition> const& function: contract->getDefinedFunctions())
 		{
 			if (function->isConstructor())
-				continue; // constructors can neither be overriden nor override anything
-			FunctionDefinition const*& override = functions[function->getName()];
+				continue; // constructors can neither be overridden nor override anything
+			string const& name = function->getName();
+			if (modifiers.count(name))
+				BOOST_THROW_EXCEPTION(modifiers[name]->createTypeError("Override changes function to modifier."));
+			FunctionDefinition const*& override = functions[name];
 			if (!override)
 				override = function.get();
 			else if (override->isPublic() != function->isPublic() ||
@@ -105,6 +120,18 @@ void ContractDefinition::checkIllegalOverrides() const
 					 FunctionType(*override) != FunctionType(*function))
 				BOOST_THROW_EXCEPTION(override->createTypeError("Override changes extended function signature."));
 		}
+		for (ASTPointer<ModifierDefinition> const& modifier: contract->getFunctionModifiers())
+		{
+			string const& name = modifier->getName();
+			if (functions.count(name))
+				BOOST_THROW_EXCEPTION(functions[name]->createTypeError("Override changes modifier to function."));
+			ModifierDefinition const*& override = modifiers[name];
+			if (!override)
+				override = modifier.get();
+			else if (ModifierType(*override) != ModifierType(*modifier))
+				BOOST_THROW_EXCEPTION(override->createTypeError("Override changes modifier signature."));
+		}
+	}
 }
 
 vector<pair<FixedHash<4>, FunctionDefinition const*>> const& ContractDefinition::getInterfaceFunctionList() const
@@ -141,6 +168,11 @@ void InheritanceSpecifier::checkTypeRequirements()
 			BOOST_THROW_EXCEPTION(createTypeError("Invalid type for argument in constructer call."));
 }
 
+TypePointer StructDefinition::getType(ContractDefinition const*) const
+{
+	return make_shared<TypeType>(make_shared<StructType>(*this));
+}
+
 void StructDefinition::checkMemberTypes() const
 {
 	for (ASTPointer<VariableDeclaration> const& member: getMembers())
@@ -169,11 +201,18 @@ void StructDefinition::checkRecursion() const
 	}
 }
 
+TypePointer FunctionDefinition::getType(ContractDefinition const*) const
+{
+	return make_shared<FunctionType>(*this);
+}
+
 void FunctionDefinition::checkTypeRequirements()
 {
 	for (ASTPointer<VariableDeclaration> const& var: getParameters() + getReturnParameters())
 		if (!var->getType()->canLiveOutsideStorage())
 			BOOST_THROW_EXCEPTION(var->createTypeError("Type is required to live outside storage."));
+	for (ASTPointer<ModifierInvocation> const& modifier: m_functionModifiers)
+		modifier->checkTypeRequirements();
 
 	m_body->checkTypeRequirements();
 }
@@ -183,6 +222,40 @@ string FunctionDefinition::getCanonicalSignature() const
 	return getName() + FunctionType(*this).getCanonicalSignature();
 }
 
+Declaration::LValueType VariableDeclaration::getLValueType() const
+{
+	if (dynamic_cast<FunctionDefinition const*>(getScope()) || dynamic_cast<ModifierDefinition const*>(getScope()))
+		return Declaration::LValueType::LOCAL;
+	else
+		return Declaration::LValueType::STORAGE;
+}
+
+TypePointer ModifierDefinition::getType(ContractDefinition const*) const
+{
+	return make_shared<ModifierType>(*this);
+}
+
+void ModifierDefinition::checkTypeRequirements()
+{
+	m_body->checkTypeRequirements();
+}
+
+void ModifierInvocation::checkTypeRequirements()
+{
+	m_modifierName->checkTypeRequirements();
+	for (ASTPointer<Expression> const& argument: m_arguments)
+		argument->checkTypeRequirements();
+
+	ModifierDefinition const* modifier = dynamic_cast<ModifierDefinition const*>(m_modifierName->getReferencedDeclaration());
+	solAssert(modifier, "Function modifier not found.");
+	vector<ASTPointer<VariableDeclaration>> const& parameters = modifier->getParameters();
+	if (parameters.size() != m_arguments.size())
+		BOOST_THROW_EXCEPTION(createTypeError("Wrong argument count for modifier invocation."));
+	for (size_t i = 0; i < m_arguments.size(); ++i)
+		if (!m_arguments[i]->getType()->isImplicitlyConvertibleTo(*parameters[i]->getType()))
+			BOOST_THROW_EXCEPTION(createTypeError("Invalid type for argument in modifier invocation."));
+}
+
 void Block::checkTypeRequirements()
 {
 	for (shared_ptr<Statement> const& statement: m_statements)
@@ -218,7 +291,8 @@ void Return::checkTypeRequirements()
 {
 	if (!m_expression)
 		return;
-	solAssert(m_returnParameters, "Return parameters not assigned.");
+	if (!m_returnParameters)
+		BOOST_THROW_EXCEPTION(createTypeError("Return arguments not allowed."));
 	if (m_returnParameters->getParameters().size() != 1)
 		BOOST_THROW_EXCEPTION(createTypeError("Different number of arguments in return statement "
 											  "than in returns declaration."));
@@ -394,7 +468,7 @@ void MemberAccess::checkTypeRequirements()
 		BOOST_THROW_EXCEPTION(createTypeError("Member \"" + *m_memberName + "\" not found or not "
 											  "visible in " + type.toString()));
 	//@todo later, this will not always be STORAGE
-	m_lvalue = type.getCategory() == Type::Category::STRUCT ? LValueType::STORAGE : LValueType::NONE;
+	m_lvalue = type.getCategory() == Type::Category::STRUCT ? Declaration::LValueType::STORAGE : Declaration::LValueType::NONE;
 }
 
 void IndexAccess::checkTypeRequirements()
@@ -406,52 +480,17 @@ void IndexAccess::checkTypeRequirements()
 	MappingType const& type = dynamic_cast<MappingType const&>(*m_base->getType());
 	m_index->expectType(*type.getKeyType());
 	m_type = type.getValueType();
-	m_lvalue = LValueType::STORAGE;
+	m_lvalue = Declaration::LValueType::STORAGE;
 }
 
 void Identifier::checkTypeRequirements()
 {
 	solAssert(m_referencedDeclaration, "Identifier not resolved.");
 
-	VariableDeclaration const* variable = dynamic_cast<VariableDeclaration const*>(m_referencedDeclaration);
-	if (variable)
-	{
-		if (!variable->getType())
-			BOOST_THROW_EXCEPTION(createTypeError("Variable referenced before type could be determined."));
-		m_type = variable->getType();
-		m_lvalue = variable->isLocalVariable() ? LValueType::LOCAL : LValueType::STORAGE;
-		return;
-	}
-	//@todo can we unify these with TypeName::toType()?
-	StructDefinition const* structDef = dynamic_cast<StructDefinition const*>(m_referencedDeclaration);
-	if (structDef)
-	{
-		// note that we do not have a struct type here
-		m_type = make_shared<TypeType>(make_shared<StructType>(*structDef));
-		return;
-	}
-	FunctionDefinition const* functionDef = dynamic_cast<FunctionDefinition const*>(m_referencedDeclaration);
-	if (functionDef)
-	{
-		// a function reference is not a TypeType, because calling a TypeType converts to the type.
-		// Calling a function (e.g. function(12), otherContract.function(34)) does not do a type
-		// conversion.
-		m_type = make_shared<FunctionType>(*functionDef);
-		return;
-	}
-	ContractDefinition const* contractDef = dynamic_cast<ContractDefinition const*>(m_referencedDeclaration);
-	if (contractDef)
-	{
-		m_type = make_shared<TypeType>(make_shared<ContractType>(*contractDef), m_currentContract);
-		return;
-	}
-	MagicVariableDeclaration const* magicVariable = dynamic_cast<MagicVariableDeclaration const*>(m_referencedDeclaration);
-	if (magicVariable)
-	{
-		m_type = magicVariable->getType();
-		return;
-	}
-	BOOST_THROW_EXCEPTION(InternalCompilerError() << errinfo_comment("Declaration reference of unknown/forbidden type."));
+	m_lvalue = m_referencedDeclaration->getLValueType();
+	m_type = m_referencedDeclaration->getType(m_currentContract);
+	if (!m_type)
+		BOOST_THROW_EXCEPTION(createTypeError("Declaration referenced before type could be determined."));
 }
 
 void ElementaryTypeNameExpression::checkTypeRequirements()
diff --git a/AST.h b/AST.h
index 754e9254..e047f976 100755
--- a/AST.h
+++ b/AST.h
@@ -132,6 +132,8 @@ private:
 class Declaration: public ASTNode
 {
 public:
+	enum class LValueType { NONE, LOCAL, STORAGE };
+
 	Declaration(Location const& _location, ASTPointer<ASTString> const& _name):
 		ASTNode(_location), m_name(_name), m_scope(nullptr) {}
 
@@ -142,6 +144,13 @@ public:
 	Declaration const* getScope() const { return m_scope; }
 	void setScope(Declaration const* _scope) { m_scope = _scope; }
 
+	/// @returns the type of expressions referencing this declaration.
+	/// The current contract has to be given since this context can change the type, especially of
+	/// contract types.
+	virtual TypePointer getType(ContractDefinition const* m_currentContract = nullptr) const = 0;
+	/// @returns the lvalue type of expressions referencing this declaration
+	virtual LValueType getLValueType() const { return LValueType::NONE; }
+
 private:
 	ASTPointer<ASTString> m_name;
 	Declaration const* m_scope;
@@ -161,12 +170,14 @@ public:
 					   std::vector<ASTPointer<InheritanceSpecifier>> const& _baseContracts,
 					   std::vector<ASTPointer<StructDefinition>> const& _definedStructs,
 					   std::vector<ASTPointer<VariableDeclaration>> const& _stateVariables,
-					   std::vector<ASTPointer<FunctionDefinition>> const& _definedFunctions):
+					   std::vector<ASTPointer<FunctionDefinition>> const& _definedFunctions,
+					   std::vector<ASTPointer<ModifierDefinition>> const& _functionModifiers):
 		Declaration(_location, _name),
 		m_baseContracts(_baseContracts),
 		m_definedStructs(_definedStructs),
 		m_stateVariables(_stateVariables),
 		m_definedFunctions(_definedFunctions),
+		m_functionModifiers(_functionModifiers),
 		m_documentation(_documentation)
 	{}
 
@@ -176,8 +187,11 @@ public:
 	std::vector<ASTPointer<InheritanceSpecifier>> const& getBaseContracts() const { return m_baseContracts; }
 	std::vector<ASTPointer<StructDefinition>> const& getDefinedStructs() const { return m_definedStructs; }
 	std::vector<ASTPointer<VariableDeclaration>> const& getStateVariables() const { return m_stateVariables; }
+	std::vector<ASTPointer<ModifierDefinition>> const& getFunctionModifiers() const { return m_functionModifiers; }
 	std::vector<ASTPointer<FunctionDefinition>> const& getDefinedFunctions() const { return m_definedFunctions; }
 
+	virtual TypePointer getType(ContractDefinition const* m_currentContract) const override;
+
 	/// Checks that there are no illegal overrides, that the constructor does not have a "returns"
 	/// and calls checkTypeRequirements on all its functions.
 	void checkTypeRequirements();
@@ -207,6 +221,7 @@ private:
 	std::vector<ASTPointer<StructDefinition>> m_definedStructs;
 	std::vector<ASTPointer<VariableDeclaration>> m_stateVariables;
 	std::vector<ASTPointer<FunctionDefinition>> m_definedFunctions;
+	std::vector<ASTPointer<ModifierDefinition>> m_functionModifiers;
 	ASTPointer<ASTString> m_documentation;
 
 	std::vector<ContractDefinition const*> m_linearizedBaseContracts;
@@ -245,6 +260,8 @@ public:
 
 	std::vector<ASTPointer<VariableDeclaration>> const& getMembers() const { return m_members; }
 
+	virtual TypePointer getType(ContractDefinition const*) const override;
+
 	/// Checks that the members do not include any recursive structs and have valid types
 	/// (e.g. no functions).
 	void checkValidityOfMembers() const;
@@ -276,7 +293,20 @@ private:
 	std::vector<ASTPointer<VariableDeclaration>> m_parameters;
 };
 
-class FunctionDefinition: public Declaration
+/**
+ * Abstract class that is added to each AST node that can store local variables.
+ */
+class VariableScope
+{
+public:
+	void addLocalVariable(VariableDeclaration const& _localVariable) { m_localVariables.push_back(&_localVariable); }
+	std::vector<VariableDeclaration const*> const& getLocalVariables() const { return m_localVariables; }
+
+private:
+	std::vector<VariableDeclaration const*> m_localVariables;
+};
+
+class FunctionDefinition: public Declaration, public VariableScope
 {
 public:
 	FunctionDefinition(Location const& _location, ASTPointer<ASTString> const& _name,
@@ -285,11 +315,13 @@ public:
 					ASTPointer<ASTString> const& _documentation,
 					ASTPointer<ParameterList> const& _parameters,
 					bool _isDeclaredConst,
+					std::vector<ASTPointer<ModifierInvocation>> const& _modifiers,
 					ASTPointer<ParameterList> const& _returnParameters,
 					ASTPointer<Block> const& _body):
 	Declaration(_location, _name), m_isPublic(_isPublic), m_isConstructor(_isConstructor),
 	m_parameters(_parameters),
 	m_isDeclaredConst(_isDeclaredConst),
+	m_functionModifiers(_modifiers),
 	m_returnParameters(_returnParameters),
 	m_body(_body),
 	m_documentation(_documentation)
@@ -301,6 +333,7 @@ public:
 	bool isPublic() const { return m_isPublic; }
 	bool isConstructor() const { return m_isConstructor; }
 	bool isDeclaredConst() const { return m_isDeclaredConst; }
+	std::vector<ASTPointer<ModifierInvocation>> const& getModifiers() const { return m_functionModifiers; }
 	std::vector<ASTPointer<VariableDeclaration>> const& getParameters() const { return m_parameters->getParameters(); }
 	ParameterList const& getParameterList() const { return *m_parameters; }
 	std::vector<ASTPointer<VariableDeclaration>> const& getReturnParameters() const { return m_returnParameters->getParameters(); }
@@ -310,8 +343,7 @@ public:
 	/// Can contain a nullptr in which case indicates absence of documentation
 	ASTPointer<ASTString> const& getDocumentation() const { return m_documentation; }
 
-	void addLocalVariable(VariableDeclaration const& _localVariable) { m_localVariables.push_back(&_localVariable); }
-	std::vector<VariableDeclaration const*> const& getLocalVariables() const { return m_localVariables; }
+	virtual TypePointer getType(ContractDefinition const*) const override;
 
 	/// Checks that all parameters have allowed types and calls checkTypeRequirements on the body.
 	void checkTypeRequirements();
@@ -326,11 +358,10 @@ private:
 	bool m_isConstructor;
 	ASTPointer<ParameterList> m_parameters;
 	bool m_isDeclaredConst;
+	std::vector<ASTPointer<ModifierInvocation>> m_functionModifiers;
 	ASTPointer<ParameterList> m_returnParameters;
 	ASTPointer<Block> m_body;
 	ASTPointer<ASTString> m_documentation;
-
-	std::vector<VariableDeclaration const*> m_localVariables;
 };
 
 /**
@@ -350,10 +381,10 @@ public:
 
 	/// Returns the declared or inferred type. Can be an empty pointer if no type was explicitly
 	/// declared and there is no assignment to the variable that fixes the type.
-	std::shared_ptr<Type const> const& getType() const { return m_type; }
+	TypePointer getType(ContractDefinition const* = nullptr) const { return m_type; }
 	void setType(std::shared_ptr<Type const> const& _type) { m_type = _type; }
 
-	bool isLocalVariable() const { return !!dynamic_cast<FunctionDefinition const*>(getScope()); }
+	virtual LValueType getLValueType() const override;
 
 private:
 	ASTPointer<TypeName> m_typeName; ///< can be empty ("var")
@@ -362,6 +393,64 @@ private:
 };
 
 /**
+ * Definition of a function modifier.
+ */
+class ModifierDefinition: public Declaration, public VariableScope
+{
+public:
+	ModifierDefinition(Location const& _location,
+					   ASTPointer<ASTString> const& _name,
+					   ASTPointer<ASTString> const& _documentation,
+					   ASTPointer<ParameterList> const& _parameters,
+					   ASTPointer<Block> const& _body):
+		Declaration(_location, _name), m_documentation(_documentation),
+		m_parameters(_parameters), m_body(_body) {}
+
+	virtual void accept(ASTVisitor& _visitor) override;
+	virtual void accept(ASTConstVisitor& _visitor) const override;
+
+	std::vector<ASTPointer<VariableDeclaration>> const& getParameters() const { return m_parameters->getParameters(); }
+	ParameterList const& getParameterList() const { return *m_parameters; }
+	Block const& getBody() const { return *m_body; }
+
+	virtual TypePointer getType(ContractDefinition const* = nullptr) const override;
+
+	/// @return A shared pointer of an ASTString.
+	/// Can contain a nullptr in which case indicates absence of documentation
+	ASTPointer<ASTString> const& getDocumentation() const { return m_documentation; }
+
+	void checkTypeRequirements();
+
+private:
+	ASTPointer<ASTString> m_documentation;
+	ASTPointer<ParameterList> m_parameters;
+	ASTPointer<Block> m_body;
+};
+
+/**
+ * Invocation/usage of a modifier in a function header.
+ */
+class ModifierInvocation: public ASTNode
+{
+public:
+	ModifierInvocation(Location const& _location, ASTPointer<Identifier> const& _name,
+					   std::vector<ASTPointer<Expression>> _arguments):
+		ASTNode(_location), m_modifierName(_name), m_arguments(_arguments) {}
+
+	virtual void accept(ASTVisitor& _visitor) override;
+	virtual void accept(ASTConstVisitor& _visitor) const override;
+
+	ASTPointer<Identifier> const& getName() const { return m_modifierName; }
+	std::vector<ASTPointer<Expression>> const& getArguments() const { return m_arguments; }
+
+	void checkTypeRequirements();
+
+private:
+	ASTPointer<Identifier> m_modifierName;
+	std::vector<ASTPointer<Expression>> m_arguments;
+};
+
+/**
  * Pseudo AST node that is used as declaration for "this", "msg", "tx", "block" and the global
  * functions when such an identifier is encountered. Will never have a valid location in the source code.
  */
@@ -375,7 +464,7 @@ public:
 	virtual void accept(ASTConstVisitor&) const override { BOOST_THROW_EXCEPTION(InternalCompilerError()
 							<< errinfo_comment("MagicVariableDeclaration used inside real AST.")); }
 
-	std::shared_ptr<Type const> const& getType() const { return m_type; }
+	virtual TypePointer getType(ContractDefinition const* = nullptr) const override { return m_type; }
 
 private:
 	std::shared_ptr<Type const> m_type;
@@ -503,6 +592,21 @@ private:
 };
 
 /**
+ * Special placeholder statement denoted by "_" used in function modifiers. This is replaced by
+ * the original function when the modifier is applied.
+ */
+class PlaceholderStatement: public Statement
+{
+public:
+	PlaceholderStatement(Location const& _location): Statement(_location) {}
+
+	virtual void accept(ASTVisitor& _visitor) override;
+	virtual void accept(ASTConstVisitor& _visitor) const override;
+
+	virtual void checkTypeRequirements() override { }
+};
+
+/**
  * If-statement with an optional "else" part. Note that "else if" is modeled by having a new
  * if-statement as the false (else) body.
  */
@@ -618,12 +722,8 @@ public:
 	virtual void accept(ASTConstVisitor& _visitor) const override;
 	virtual void checkTypeRequirements() override;
 
-	void setFunctionReturnParameters(ParameterList const& _parameters) { m_returnParameters = &_parameters; }
-	ParameterList const& getFunctionReturnParameters() const
-	{
-		solAssert(m_returnParameters, "");
-		return *m_returnParameters;
-	}
+	void setFunctionReturnParameters(ParameterList const* _parameters) { m_returnParameters = _parameters; }
+	ParameterList const* getFunctionReturnParameters() const { return m_returnParameters; }
 	Expression const* getExpression() const { return m_expression.get(); }
 
 private:
@@ -686,16 +786,13 @@ private:
  */
 class Expression: public ASTNode
 {
-protected:
-	enum class LValueType { NONE, LOCAL, STORAGE };
-
 public:
-	Expression(Location const& _location): ASTNode(_location), m_lvalue(LValueType::NONE), m_lvalueRequested(false) {}
+	Expression(Location const& _location): ASTNode(_location) {}
 	virtual void checkTypeRequirements() = 0;
 
 	std::shared_ptr<Type const> const& getType() const { return m_type; }
-	bool isLValue() const { return m_lvalue != LValueType::NONE; }
-	bool isLocalLValue() const { return m_lvalue == LValueType::LOCAL; }
+	bool isLValue() const { return m_lvalue != Declaration::LValueType::NONE; }
+	bool isLocalLValue() const { return m_lvalue == Declaration::LValueType::LOCAL; }
 
 	/// Helper function, infer the type via @ref checkTypeRequirements and then check that it
 	/// is implicitly convertible to @a _expectedType. If not, throw exception.
@@ -712,9 +809,9 @@ protected:
 	std::shared_ptr<Type const> m_type;
 	//! If this expression is an lvalue (i.e. something that can be assigned to) and is stored
 	//! locally or in storage. This is set during calls to @a checkTypeRequirements()
-	LValueType m_lvalue;
+	Declaration::LValueType m_lvalue = Declaration::LValueType::NONE;
 	//! Whether the outer expression requested the address (true) or the value (false) of this expression.
-	bool m_lvalueRequested;
+	bool m_lvalueRequested = false;
 };
 
 /// Assignment, can also be a compound assignment.
diff --git a/ASTForward.h b/ASTForward.h
index da0a8812..aa5cd49c 100644
--- a/ASTForward.h
+++ b/ASTForward.h
@@ -43,6 +43,8 @@ class StructDefinition;
 class ParameterList;
 class FunctionDefinition;
 class VariableDeclaration;
+class ModifierDefinition;
+class ModifierInvocation;
 class MagicVariableDeclaration;
 class TypeName;
 class ElementaryTypeName;
@@ -50,6 +52,7 @@ class UserDefinedTypeName;
 class Mapping;
 class Statement;
 class Block;
+class PlaceholderStatement;
 class IfStatement;
 class BreakableStatement;
 class WhileStatement;
@@ -72,6 +75,8 @@ class Identifier;
 class ElementaryTypeNameExpression;
 class Literal;
 
+class VariableScope;
+
 // Used as pointers to AST nodes, to be replaced by more clever pointers, e.g. pointers which do
 // not do reference counting but point to a special memory area that is completely released
 // explicitly.
diff --git a/ASTJsonConverter.cpp b/ASTJsonConverter.cpp
index 04ddee0a..d9332990 100644
--- a/ASTJsonConverter.cpp
+++ b/ASTJsonConverter.cpp
@@ -118,9 +118,10 @@ bool ASTJsonConverter::visit(FunctionDefinition const& _node)
 
 bool ASTJsonConverter::visit(VariableDeclaration const& _node)
 {
+	bool isLocalVariable = (_node.getLValueType() == VariableDeclaration::LValueType::LOCAL);
 	addJsonNode("VariableDeclaration",
 				{   make_pair("name", _node.getName()),
-					make_pair("local", boost::lexical_cast<std::string>(_node.isLocalVariable()))},
+					make_pair("local", boost::lexical_cast<std::string>(isLocalVariable))},
 				true);
 	return true;
 }
diff --git a/ASTPrinter.cpp b/ASTPrinter.cpp
index 916fca1e..85bc8825 100644
--- a/ASTPrinter.cpp
+++ b/ASTPrinter.cpp
@@ -87,6 +87,20 @@ bool ASTPrinter::visit(VariableDeclaration const& _node)
 	return goDeeper();
 }
 
+bool ASTPrinter::visit(ModifierDefinition const& _node)
+{
+	writeLine("ModifierDefinition \"" + _node.getName() + "\"");
+	printSourcePart(_node);
+	return goDeeper();
+}
+
+bool ASTPrinter::visit(ModifierInvocation const& _node)
+{
+	writeLine("ModifierInvocation \"" + _node.getName()->getName() + "\"");
+	printSourcePart(_node);
+	return goDeeper();
+}
+
 bool ASTPrinter::visit(TypeName const& _node)
 {
 	writeLine("TypeName");
@@ -129,6 +143,13 @@ bool ASTPrinter::visit(Block const& _node)
 	return goDeeper();
 }
 
+bool ASTPrinter::visit(PlaceholderStatement const& _node)
+{
+	writeLine("PlaceholderStatement");
+	printSourcePart(_node);
+	return goDeeper();
+}
+
 bool ASTPrinter::visit(IfStatement const& _node)
 {
 	writeLine("IfStatement");
@@ -322,6 +343,16 @@ void ASTPrinter::endVisit(VariableDeclaration const&)
 	m_indentation--;
 }
 
+void ASTPrinter::endVisit(ModifierDefinition const&)
+{
+	m_indentation--;
+}
+
+void ASTPrinter::endVisit(ModifierInvocation const&)
+{
+	m_indentation--;
+}
+
 void ASTPrinter::endVisit(TypeName const&)
 {
 	m_indentation--;
@@ -352,6 +383,11 @@ void ASTPrinter::endVisit(Block const&)
 	m_indentation--;
 }
 
+void ASTPrinter::endVisit(PlaceholderStatement const&)
+{
+	m_indentation--;
+}
+
 void ASTPrinter::endVisit(IfStatement const&)
 {
 	m_indentation--;
diff --git a/ASTPrinter.h b/ASTPrinter.h
index fc5fb4ac..7f267bdf 100644
--- a/ASTPrinter.h
+++ b/ASTPrinter.h
@@ -48,12 +48,15 @@ public:
 	bool visit(ParameterList const& _node) override;
 	bool visit(FunctionDefinition const& _node) override;
 	bool visit(VariableDeclaration const& _node) override;
+	bool visit(ModifierDefinition const& _node) override;
+	bool visit(ModifierInvocation const& _node) override;
 	bool visit(TypeName const& _node) override;
 	bool visit(ElementaryTypeName const& _node) override;
 	bool visit(UserDefinedTypeName const& _node) override;
 	bool visit(Mapping const& _node) override;
 	bool visit(Statement const& _node) override;
 	bool visit(Block const& _node) override;
+	bool visit(PlaceholderStatement const& _node) override;
 	bool visit(IfStatement const& _node) override;
 	bool visit(BreakableStatement const& _node) override;
 	bool visit(WhileStatement const& _node) override;
@@ -82,12 +85,15 @@ public:
 	void endVisit(ParameterList const&) override;
 	void endVisit(FunctionDefinition const&) override;
 	void endVisit(VariableDeclaration const&) override;
+	void endVisit(ModifierDefinition const&) override;
+	void endVisit(ModifierInvocation const&) override;
 	void endVisit(TypeName const&) override;
 	void endVisit(ElementaryTypeName const&) override;
 	void endVisit(UserDefinedTypeName const&) override;
 	void endVisit(Mapping const&) override;
 	void endVisit(Statement const&) override;
 	void endVisit(Block const&) override;
+	void endVisit(PlaceholderStatement const&) override;
 	void endVisit(IfStatement const&) override;
 	void endVisit(BreakableStatement const&) override;
 	void endVisit(WhileStatement const&) override;
diff --git a/ASTVisitor.h b/ASTVisitor.h
index 33a8a338..ecab00c3 100644
--- a/ASTVisitor.h
+++ b/ASTVisitor.h
@@ -49,12 +49,15 @@ public:
 	virtual bool visit(ParameterList&) { return true; }
 	virtual bool visit(FunctionDefinition&) { return true; }
 	virtual bool visit(VariableDeclaration&) { return true; }
+	virtual bool visit(ModifierDefinition&) { return true; }
+	virtual bool visit(ModifierInvocation&) { return true; }
 	virtual bool visit(TypeName&) { return true; }
 	virtual bool visit(ElementaryTypeName&) { return true; }
 	virtual bool visit(UserDefinedTypeName&) { return true; }
 	virtual bool visit(Mapping&) { return true; }
 	virtual bool visit(Statement&) { return true; }
 	virtual bool visit(Block&) { return true; }
+	virtual bool visit(PlaceholderStatement&) { return true; }
 	virtual bool visit(IfStatement&) { return true; }
 	virtual bool visit(BreakableStatement&) { return true; }
 	virtual bool visit(WhileStatement&) { return true; }
@@ -85,12 +88,15 @@ public:
 	virtual void endVisit(ParameterList&) { }
 	virtual void endVisit(FunctionDefinition&) { }
 	virtual void endVisit(VariableDeclaration&) { }
+	virtual void endVisit(ModifierDefinition&) { }
+	virtual void endVisit(ModifierInvocation&) { }
 	virtual void endVisit(TypeName&) { }
 	virtual void endVisit(ElementaryTypeName&) { }
 	virtual void endVisit(UserDefinedTypeName&) { }
 	virtual void endVisit(Mapping&) { }
 	virtual void endVisit(Statement&) { }
 	virtual void endVisit(Block&) { }
+	virtual void endVisit(PlaceholderStatement&) { }
 	virtual void endVisit(IfStatement&) { }
 	virtual void endVisit(BreakableStatement&) { }
 	virtual void endVisit(WhileStatement&) { }
@@ -125,12 +131,15 @@ public:
 	virtual bool visit(ParameterList const&) { return true; }
 	virtual bool visit(FunctionDefinition const&) { return true; }
 	virtual bool visit(VariableDeclaration const&) { return true; }
+	virtual bool visit(ModifierDefinition const&) { return true; }
+	virtual bool visit(ModifierInvocation const&) { return true; }
 	virtual bool visit(TypeName const&) { return true; }
 	virtual bool visit(ElementaryTypeName const&) { return true; }
 	virtual bool visit(UserDefinedTypeName const&) { return true; }
 	virtual bool visit(Mapping const&) { return true; }
 	virtual bool visit(Statement const&) { return true; }
 	virtual bool visit(Block const&) { return true; }
+	virtual bool visit(PlaceholderStatement const&) { return true; }
 	virtual bool visit(IfStatement const&) { return true; }
 	virtual bool visit(BreakableStatement const&) { return true; }
 	virtual bool visit(WhileStatement const&) { return true; }
@@ -161,12 +170,15 @@ public:
 	virtual void endVisit(ParameterList const&) { }
 	virtual void endVisit(FunctionDefinition const&) { }
 	virtual void endVisit(VariableDeclaration const&) { }
+	virtual void endVisit(ModifierDefinition const&) { }
+	virtual void endVisit(ModifierInvocation const&) { }
 	virtual void endVisit(TypeName const&) { }
 	virtual void endVisit(ElementaryTypeName const&) { }
 	virtual void endVisit(UserDefinedTypeName const&) { }
 	virtual void endVisit(Mapping const&) { }
 	virtual void endVisit(Statement const&) { }
 	virtual void endVisit(Block const&) { }
+	virtual void endVisit(PlaceholderStatement const&) { }
 	virtual void endVisit(IfStatement const&) { }
 	virtual void endVisit(BreakableStatement const&) { }
 	virtual void endVisit(WhileStatement const&) { }
diff --git a/AST_accept.h b/AST_accept.h
index b77cfe1c..481b150b 100644
--- a/AST_accept.h
+++ b/AST_accept.h
@@ -65,6 +65,7 @@ void ContractDefinition::accept(ASTVisitor& _visitor)
 		listAccept(m_definedStructs, _visitor);
 		listAccept(m_stateVariables, _visitor);
 		listAccept(m_definedFunctions, _visitor);
+		listAccept(m_functionModifiers, _visitor);
 	}
 	_visitor.endVisit(*this);
 }
@@ -77,6 +78,7 @@ void ContractDefinition::accept(ASTConstVisitor& _visitor) const
 		listAccept(m_definedStructs, _visitor);
 		listAccept(m_stateVariables, _visitor);
 		listAccept(m_definedFunctions, _visitor);
+		listAccept(m_functionModifiers, _visitor);
 	}
 	_visitor.endVisit(*this);
 }
@@ -142,6 +144,7 @@ void FunctionDefinition::accept(ASTVisitor& _visitor)
 		m_parameters->accept(_visitor);
 		if (m_returnParameters)
 			m_returnParameters->accept(_visitor);
+		listAccept(m_functionModifiers, _visitor);
 		m_body->accept(_visitor);
 	}
 	_visitor.endVisit(*this);
@@ -154,6 +157,7 @@ void FunctionDefinition::accept(ASTConstVisitor& _visitor) const
 		m_parameters->accept(_visitor);
 		if (m_returnParameters)
 			m_returnParameters->accept(_visitor);
+		listAccept(m_functionModifiers, _visitor);
 		m_body->accept(_visitor);
 	}
 	_visitor.endVisit(*this);
@@ -175,6 +179,46 @@ void VariableDeclaration::accept(ASTConstVisitor& _visitor) const
 	_visitor.endVisit(*this);
 }
 
+void ModifierDefinition::accept(ASTVisitor& _visitor)
+{
+	if (_visitor.visit(*this))
+	{
+		m_parameters->accept(_visitor);
+		m_body->accept(_visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
+void ModifierDefinition::accept(ASTConstVisitor& _visitor) const
+{
+	if (_visitor.visit(*this))
+	{
+		m_parameters->accept(_visitor);
+		m_body->accept(_visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
+void ModifierInvocation::accept(ASTVisitor& _visitor)
+{
+	if (_visitor.visit(*this))
+	{
+		m_modifierName->accept(_visitor);
+		listAccept(m_arguments, _visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
+void ModifierInvocation::accept(ASTConstVisitor& _visitor) const
+{
+	if (_visitor.visit(*this))
+	{
+		m_modifierName->accept(_visitor);
+		listAccept(m_arguments, _visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
 void TypeName::accept(ASTVisitor& _visitor)
 {
 	_visitor.visit(*this);
@@ -245,6 +289,18 @@ void Block::accept(ASTConstVisitor& _visitor) const
 	_visitor.endVisit(*this);
 }
 
+void PlaceholderStatement::accept(ASTVisitor& _visitor)
+{
+	_visitor.visit(*this);
+	_visitor.endVisit(*this);
+}
+
+void PlaceholderStatement::accept(ASTConstVisitor& _visitor) const
+{
+	_visitor.visit(*this);
+	_visitor.endVisit(*this);
+}
+
 void IfStatement::accept(ASTVisitor& _visitor)
 {
 	if (_visitor.visit(*this))
diff --git a/CallGraph.cpp b/CallGraph.cpp
index a671796b..5f8fc547 100644
--- a/CallGraph.cpp
+++ b/CallGraph.cpp
@@ -33,7 +33,11 @@ namespace solidity
 
 void CallGraph::addNode(ASTNode const& _node)
 {
-	_node.accept(*this);
+	if (!m_nodesSeen.count(&_node))
+	{
+		m_workQueue.push(&_node);
+		m_nodesSeen.insert(&_node);
+	}
 }
 
 set<FunctionDefinition const*> const& CallGraph::getCalls()
@@ -53,20 +57,26 @@ void CallGraph::computeCallGraph()
 
 bool CallGraph::visit(Identifier const& _identifier)
 {
-	FunctionDefinition const* fun = dynamic_cast<FunctionDefinition const*>(_identifier.getReferencedDeclaration());
-	if (fun)
+	if (auto fun = dynamic_cast<FunctionDefinition const*>(_identifier.getReferencedDeclaration()))
 	{
-		if (m_overrideResolver)
-			fun = (*m_overrideResolver)(fun->getName());
+		if (m_functionOverrideResolver)
+			fun = (*m_functionOverrideResolver)(fun->getName());
 		solAssert(fun, "Error finding override for function " + fun->getName());
-		addFunction(*fun);
+		addNode(*fun);
+	}
+	if (auto modifier = dynamic_cast<ModifierDefinition const*>(_identifier.getReferencedDeclaration()))
+	{
+		if (m_modifierOverrideResolver)
+			modifier = (*m_modifierOverrideResolver)(modifier->getName());
+		solAssert(modifier, "Error finding override for modifier " + modifier->getName());
+		addNode(*modifier);
 	}
 	return true;
 }
 
 bool CallGraph::visit(FunctionDefinition const& _function)
 {
-	addFunction(_function);
+	m_functionsSeen.insert(&_function);
 	return true;
 }
 
@@ -83,7 +93,7 @@ bool CallGraph::visit(MemberAccess const& _memberAccess)
 			for (ASTPointer<FunctionDefinition> const& function: contract.getDefinedFunctions())
 				if (function->getName() == _memberAccess.getMemberName())
 				{
-					addFunction(*function);
+					addNode(*function);
 					return true;
 				}
 		}
@@ -91,14 +101,5 @@ bool CallGraph::visit(MemberAccess const& _memberAccess)
 	return true;
 }
 
-void CallGraph::addFunction(FunctionDefinition const& _function)
-{
-	if (!m_functionsSeen.count(&_function))
-	{
-		m_functionsSeen.insert(&_function);
-		m_workQueue.push(&_function);
-	}
-}
-
 }
 }
diff --git a/CallGraph.h b/CallGraph.h
index 90176e7e..9af5cdf9 100644
--- a/CallGraph.h
+++ b/CallGraph.h
@@ -39,9 +39,13 @@ namespace solidity
 class CallGraph: private ASTConstVisitor
 {
 public:
-	using OverrideResolver = std::function<FunctionDefinition const*(std::string const&)>;
+	using FunctionOverrideResolver = std::function<FunctionDefinition const*(std::string const&)>;
+	using ModifierOverrideResolver = std::function<ModifierDefinition const*(std::string const&)>;
 
-	CallGraph(OverrideResolver const& _overrideResolver): m_overrideResolver(&_overrideResolver) {}
+	CallGraph(FunctionOverrideResolver const& _functionOverrideResolver,
+			  ModifierOverrideResolver const& _modifierOverrideResolver):
+		m_functionOverrideResolver(&_functionOverrideResolver),
+		m_modifierOverrideResolver(&_modifierOverrideResolver) {}
 
 	void addNode(ASTNode const& _node);
 
@@ -53,11 +57,12 @@ private:
 	virtual bool visit(MemberAccess const& _memberAccess) override;
 
 	void computeCallGraph();
-	void addFunction(FunctionDefinition const& _function);
 
-	OverrideResolver const* m_overrideResolver;
+	FunctionOverrideResolver const* m_functionOverrideResolver;
+	ModifierOverrideResolver const* m_modifierOverrideResolver;
+	std::set<ASTNode const*> m_nodesSeen;
 	std::set<FunctionDefinition const*> m_functionsSeen;
-	std::queue<FunctionDefinition const*> m_workQueue;
+	std::queue<ASTNode const*> m_workQueue;
 };
 
 }
diff --git a/Compiler.cpp b/Compiler.cpp
index 5a434a71..5190f93f 100644
--- a/Compiler.cpp
+++ b/Compiler.cpp
@@ -42,9 +42,13 @@ void Compiler::compileContract(ContractDefinition const& _contract,
 	initializeContext(_contract, _contracts);
 
 	for (ContractDefinition const* contract: _contract.getLinearizedBaseContracts())
+	{
 		for (ASTPointer<FunctionDefinition> const& function: contract->getDefinedFunctions())
 			if (!function->isConstructor())
 				m_context.addFunction(*function);
+		for (ASTPointer<ModifierDefinition> const& modifier: contract->getFunctionModifiers())
+			m_context.addModifier(*modifier);
+	}
 
 	appendFunctionSelector(_contract);
 	for (ContractDefinition const* contract: _contract.getLinearizedBaseContracts())
@@ -67,6 +71,13 @@ void Compiler::initializeContext(ContractDefinition const& _contract,
 
 void Compiler::packIntoContractCreator(ContractDefinition const& _contract, CompilerContext const& _runtimeContext)
 {
+	std::vector<ContractDefinition const*> const& bases = _contract.getLinearizedBaseContracts();
+
+	// Make all modifiers known to the context.
+	for (ContractDefinition const* contract: bases)
+		for (ASTPointer<ModifierDefinition> const& modifier: contract->getFunctionModifiers())
+			m_context.addModifier(*modifier);
+
 	// arguments for base constructors, filled in derived-to-base order
 	map<ContractDefinition const*, vector<ASTPointer<Expression>> const*> baseArguments;
 	set<FunctionDefinition const*> neededFunctions;
@@ -74,7 +85,6 @@ void Compiler::packIntoContractCreator(ContractDefinition const& _contract, Comp
 
 	// Determine the arguments that are used for the base constructors and also which functions
 	// are needed at compile time.
-	std::vector<ContractDefinition const*> const& bases = _contract.getLinearizedBaseContracts();
 	for (ContractDefinition const* contract: bases)
 	{
 		if (FunctionDefinition const* constructor = contract->getConstructor())
@@ -93,7 +103,7 @@ void Compiler::packIntoContractCreator(ContractDefinition const& _contract, Comp
 		}
 	}
 
-	auto overrideResolver = [&](string const& _name) -> FunctionDefinition const*
+	auto functionOverrideResolver = [&](string const& _name) -> FunctionDefinition const*
 	{
 		for (ContractDefinition const* contract: bases)
 			for (ASTPointer<FunctionDefinition> const& function: contract->getDefinedFunctions())
@@ -101,21 +111,26 @@ void Compiler::packIntoContractCreator(ContractDefinition const& _contract, Comp
 					return function.get();
 		return nullptr;
 	};
+	auto modifierOverrideResolver = [&](string const& _name) -> ModifierDefinition const*
+	{
+		return &m_context.getFunctionModifier(_name);
+	};
 
-	neededFunctions = getFunctionsCalled(nodesUsedInConstructors, overrideResolver);
+	neededFunctions = getFunctionsCalled(nodesUsedInConstructors, functionOverrideResolver,
+										 modifierOverrideResolver);
 
 	// First add all overrides (or the functions themselves if there is no override)
 	for (FunctionDefinition const* fun: neededFunctions)
 	{
 		FunctionDefinition const* override = nullptr;
 		if (!fun->isConstructor())
-			override = overrideResolver(fun->getName());
+			override = functionOverrideResolver(fun->getName());
 		if (!!override && neededFunctions.count(override))
 			m_context.addFunction(*override);
 	}
 	// now add the rest
 	for (FunctionDefinition const* fun: neededFunctions)
-		if (fun->isConstructor() || overrideResolver(fun->getName()) != fun)
+		if (fun->isConstructor() || functionOverrideResolver(fun->getName()) != fun)
 			m_context.addFunction(*fun);
 
 	// Call constructors in base-to-derived order.
@@ -150,11 +165,7 @@ void Compiler::appendBaseConstructorCall(FunctionDefinition const& _constructor,
 	FunctionType constructorType(_constructor);
 	eth::AssemblyItem returnLabel = m_context.pushNewTag();
 	for (unsigned i = 0; i < _arguments.size(); ++i)
-	{
-		compileExpression(*_arguments[i]);
-		ExpressionCompiler::appendTypeConversion(m_context, *_arguments[i]->getType(),
-												 *constructorType.getParameterTypes()[i]);
-	}
+		compileExpression(*_arguments[i], constructorType.getParameterTypes()[i]);
 	m_context.appendJumpTo(m_context.getFunctionEntryLabel(_constructor));
 	m_context << returnLabel;
 }
@@ -179,9 +190,10 @@ void Compiler::appendConstructorCall(FunctionDefinition const& _constructor)
 }
 
 set<FunctionDefinition const*> Compiler::getFunctionsCalled(set<ASTNode const*> const& _nodes,
-						function<FunctionDefinition const*(string const&)> const& _resolveOverrides)
+						function<FunctionDefinition const*(string const&)> const& _resolveFunctionOverrides,
+						function<ModifierDefinition const*(string const&)> const& _resolveModifierOverrides)
 {
-	CallGraph callgraph(_resolveOverrides);
+	CallGraph callgraph(_resolveFunctionOverrides, _resolveModifierOverrides);
 	for (ASTNode const* node: _nodes)
 		callgraph.addNode(*node);
 	return callgraph.getCalls();
@@ -280,20 +292,28 @@ bool Compiler::visit(FunctionDefinition const& _function)
 	m_returnTag = m_context.newTag();
 	m_breakTags.clear();
 	m_continueTags.clear();
+	m_stackCleanupForReturn = 0;
+	m_currentFunction = &_function;
+	m_modifierDepth = 0;
 
 	m_context << m_context.getFunctionEntryLabel(_function);
 
 	// stack upon entry: [return address] [arg0] [arg1] ... [argn]
 	// reserve additional slots: [retarg0] ... [retargm] [localvar0] ... [localvarp]
 
+	unsigned parametersSize = CompilerUtils::getSizeOnStack(_function.getParameters());
+	m_context.adjustStackOffset(parametersSize);
 	for (ASTPointer<VariableDeclaration const> const& variable: _function.getParameters())
-		m_context.addVariable(*variable);
+	{
+		m_context.addVariable(*variable, parametersSize);
+		parametersSize -= variable->getType()->getSizeOnStack();
+	}
 	for (ASTPointer<VariableDeclaration const> const& variable: _function.getReturnParameters())
 		m_context.addAndInitializeVariable(*variable);
 	for (VariableDeclaration const* localVariable: _function.getLocalVariables())
 		m_context.addAndInitializeVariable(*localVariable);
 
-	_function.getBody().accept(*this);
+	appendModifierOrFunctionCode();
 
 	m_context << m_returnTag;
 
@@ -420,13 +440,15 @@ bool Compiler::visit(Return const& _return)
 	//@todo modifications are needed to make this work with functions returning multiple values
 	if (Expression const* expression = _return.getExpression())
 	{
-		compileExpression(*expression);
-		VariableDeclaration const& firstVariable = *_return.getFunctionReturnParameters().getParameters().front();
-		ExpressionCompiler::appendTypeConversion(m_context, *expression->getType(), *firstVariable.getType());
-
+		solAssert(_return.getFunctionReturnParameters(), "Invalid return parameters pointer.");
+		VariableDeclaration const& firstVariable = *_return.getFunctionReturnParameters()->getParameters().front();
+		compileExpression(*expression, firstVariable.getType());
 		CompilerUtils(m_context).moveToStackVariable(firstVariable);
 	}
+	for (unsigned i = 0; i < m_stackCleanupForReturn; ++i)
+		m_context << eth::Instruction::POP;
 	m_context.appendJumpTo(m_returnTag);
+	m_context.adjustStackOffset(m_stackCleanupForReturn);
 	return false;
 }
 
@@ -434,10 +456,7 @@ bool Compiler::visit(VariableDefinition const& _variableDefinition)
 {
 	if (Expression const* expression = _variableDefinition.getExpression())
 	{
-		compileExpression(*expression);
-		ExpressionCompiler::appendTypeConversion(m_context,
-												 *expression->getType(),
-												 *_variableDefinition.getDeclaration().getType());
+		compileExpression(*expression, _variableDefinition.getDeclaration().getType());
 		CompilerUtils(m_context).moveToStackVariable(_variableDefinition.getDeclaration());
 	}
 	return false;
@@ -451,9 +470,51 @@ bool Compiler::visit(ExpressionStatement const& _expressionStatement)
 	return false;
 }
 
-void Compiler::compileExpression(Expression const& _expression)
+bool Compiler::visit(PlaceholderStatement const&)
+{
+	++m_modifierDepth;
+	appendModifierOrFunctionCode();
+	--m_modifierDepth;
+	return true;
+}
+
+void Compiler::appendModifierOrFunctionCode()
+{
+	solAssert(m_currentFunction, "");
+	if (m_modifierDepth >= m_currentFunction->getModifiers().size())
+		m_currentFunction->getBody().accept(*this);
+	else
+	{
+		ASTPointer<ModifierInvocation> const& modifierInvocation = m_currentFunction->getModifiers()[m_modifierDepth];
+
+		ModifierDefinition const& modifier = m_context.getFunctionModifier(modifierInvocation->getName()->getName());
+		solAssert(modifier.getParameters().size() == modifierInvocation->getArguments().size(), "");
+		for (unsigned i = 0; i < modifier.getParameters().size(); ++i)
+		{
+			m_context.addVariable(*modifier.getParameters()[i]);
+			compileExpression(*modifierInvocation->getArguments()[i],
+							  modifier.getParameters()[i]->getType());
+		}
+		for (VariableDeclaration const* localVariable: modifier.getLocalVariables())
+			m_context.addAndInitializeVariable(*localVariable);
+
+		unsigned const c_stackSurplus = CompilerUtils::getSizeOnStack(modifier.getParameters()) +
+										CompilerUtils::getSizeOnStack(modifier.getLocalVariables());
+		m_stackCleanupForReturn += c_stackSurplus;
+
+		modifier.getBody().accept(*this);
+
+		for (unsigned i = 0; i < c_stackSurplus; ++i)
+			m_context << eth::Instruction::POP;
+		m_stackCleanupForReturn -= c_stackSurplus;
+	}
+}
+
+void Compiler::compileExpression(Expression const& _expression, TypePointer const& _targetType)
 {
 	ExpressionCompiler::compileExpression(m_context, _expression, m_optimize);
+	if (_targetType)
+		ExpressionCompiler::appendTypeConversion(m_context, *_expression.getType(), *_targetType);
 }
 
 }
diff --git a/Compiler.h b/Compiler.h
index 2bae6b39..b6547646 100644
--- a/Compiler.h
+++ b/Compiler.h
@@ -31,7 +31,8 @@ namespace solidity {
 class Compiler: private ASTConstVisitor
 {
 public:
-	explicit Compiler(bool _optimize = false): m_optimize(_optimize), m_context(), m_returnTag(m_context.newTag()) {}
+	explicit Compiler(bool _optimize = false): m_optimize(_optimize), m_context(),
+		m_returnTag(m_context.newTag()) {}
 
 	void compileContract(ContractDefinition const& _contract,
 						 std::map<ContractDefinition const*, bytes const*> const& _contracts);
@@ -52,7 +53,8 @@ private:
 	/// Recursively searches the call graph and returns all functions referenced inside _nodes.
 	/// _resolveOverride is called to resolve virtual function overrides.
 	std::set<FunctionDefinition const*> getFunctionsCalled(std::set<ASTNode const*> const& _nodes,
-					std::function<FunctionDefinition const*(std::string const&)> const& _resolveOverride);
+					std::function<FunctionDefinition const*(std::string const&)> const& _resolveFunctionOverride,
+					std::function<ModifierDefinition const*(std::string const&)> const& _resolveModifierOverride);
 	void appendFunctionSelector(ContractDefinition const& _contract);
 	/// Creates code that unpacks the arguments for the given function, from memory if
 	/// @a _fromMemory is true, otherwise from call data. @returns the size of the data in bytes.
@@ -70,8 +72,13 @@ private:
 	virtual bool visit(Return const& _return) override;
 	virtual bool visit(VariableDefinition const& _variableDefinition) override;
 	virtual bool visit(ExpressionStatement const& _expressionStatement) override;
+	virtual bool visit(PlaceholderStatement const&) override;
 
-	void compileExpression(Expression const& _expression);
+	/// Appends one layer of function modifier code of the current function, or the function
+	/// body itself if the last modifier was reached.
+	void appendModifierOrFunctionCode();
+
+	void compileExpression(Expression const& _expression, TypePointer const& _targetType = TypePointer());
 
 	bool const m_optimize;
 	CompilerContext m_context;
@@ -79,6 +86,9 @@ private:
 	std::vector<eth::AssemblyItem> m_breakTags; ///< tag to jump to for a "break" statement
 	std::vector<eth::AssemblyItem> m_continueTags; ///< tag to jump to for a "continue" statement
 	eth::AssemblyItem m_returnTag; ///< tag to jump to for a "return" statement
+	unsigned m_modifierDepth = 0;
+	FunctionDefinition const* m_currentFunction;
+	unsigned m_stackCleanupForReturn; ///< this number of stack elements need to be removed before jump to m_returnTag
 };
 
 }
diff --git a/CompilerContext.cpp b/CompilerContext.cpp
index 27ec3efd..ad1877ba 100644
--- a/CompilerContext.cpp
+++ b/CompilerContext.cpp
@@ -43,10 +43,11 @@ void CompilerContext::addStateVariable(VariableDeclaration const& _declaration)
 	m_stateVariablesSize += _declaration.getType()->getStorageSize();
 }
 
-void CompilerContext::addVariable(VariableDeclaration const& _declaration)
+void CompilerContext::addVariable(VariableDeclaration const& _declaration,
+								  unsigned _offsetToCurrent)
 {
-	m_localVariables[&_declaration] = m_localVariablesSize;
-	m_localVariablesSize += _declaration.getType()->getSizeOnStack();
+	solAssert(m_asm.deposit() >= 0 && unsigned(m_asm.deposit()) >= _offsetToCurrent, "");
+	m_localVariables[&_declaration] = unsigned(m_asm.deposit()) - _offsetToCurrent;
 }
 
 void CompilerContext::addAndInitializeVariable(VariableDeclaration const& _declaration)
@@ -56,7 +57,6 @@ void CompilerContext::addAndInitializeVariable(VariableDeclaration const& _decla
 	int const size = _declaration.getType()->getSizeOnStack();
 	for (int i = 0; i < size; ++i)
 		*this << u256(0);
-	m_asm.adjustDeposit(-size);
 }
 
 void CompilerContext::addFunction(FunctionDefinition const& _function)
@@ -66,6 +66,11 @@ void CompilerContext::addFunction(FunctionDefinition const& _function)
 	m_virtualFunctionEntryLabels.insert(make_pair(_function.getName(), tag));
 }
 
+void CompilerContext::addModifier(ModifierDefinition const& _modifier)
+{
+	m_functionModifiers.insert(make_pair(_modifier.getName(),  &_modifier));
+}
+
 bytes const& CompilerContext::getCompiledContract(const ContractDefinition& _contract) const
 {
 	auto ret = m_compiledContracts.find(&_contract);
@@ -75,7 +80,7 @@ bytes const& CompilerContext::getCompiledContract(const ContractDefinition& _con
 
 bool CompilerContext::isLocalVariable(Declaration const* _declaration) const
 {
-	return m_localVariables.count(_declaration) > 0;
+	return m_localVariables.count(_declaration);
 }
 
 eth::AssemblyItem CompilerContext::getFunctionEntryLabel(FunctionDefinition const& _function) const
@@ -92,21 +97,28 @@ eth::AssemblyItem CompilerContext::getVirtualFunctionEntryLabel(FunctionDefiniti
 	return res->second.tag();
 }
 
+ModifierDefinition const& CompilerContext::getFunctionModifier(string const& _name) const
+{
+	auto res = m_functionModifiers.find(_name);
+	solAssert(res != m_functionModifiers.end(), "Function modifier override not found.");
+	return *res->second;
+}
+
 unsigned CompilerContext::getBaseStackOffsetOfVariable(Declaration const& _declaration) const
 {
 	auto res = m_localVariables.find(&_declaration);
 	solAssert(res != m_localVariables.end(), "Variable not found on stack.");
-	return m_localVariablesSize - res->second - 1;
+	return res->second;
 }
 
 unsigned CompilerContext::baseToCurrentStackOffset(unsigned _baseOffset) const
 {
-	return _baseOffset + m_asm.deposit();
+	return m_asm.deposit() - _baseOffset - 1;
 }
 
 unsigned CompilerContext::currentToBaseStackOffset(unsigned _offset) const
 {
-	return -baseToCurrentStackOffset(-_offset);
+	return m_asm.deposit() - _offset - 1;
 }
 
 u256 CompilerContext::getStorageLocationOfVariable(const Declaration& _declaration) const
diff --git a/CompilerContext.h b/CompilerContext.h
index cde992d5..d82dfe51 100644
--- a/CompilerContext.h
+++ b/CompilerContext.h
@@ -42,9 +42,11 @@ public:
 	void addMagicGlobal(MagicVariableDeclaration const& _declaration);
 	void addStateVariable(VariableDeclaration const& _declaration);
 	void startNewFunction() { m_localVariables.clear(); m_asm.setDeposit(0); }
-	void addVariable(VariableDeclaration const& _declaration);
+	void addVariable(VariableDeclaration const& _declaration, unsigned _offsetToCurrent = 0);
 	void addAndInitializeVariable(VariableDeclaration const& _declaration);
 	void addFunction(FunctionDefinition const& _function);
+	/// Adds the given modifier to the list by name if the name is not present already.
+	void addModifier(ModifierDefinition const& _modifier);
 
 	void setCompiledContracts(std::map<ContractDefinition const*, bytes const*> const& _contracts) { m_compiledContracts = _contracts; }
 	bytes const& getCompiledContract(ContractDefinition const& _contract) const;
@@ -59,7 +61,8 @@ public:
 	eth::AssemblyItem getFunctionEntryLabel(FunctionDefinition const& _function) const;
 	/// @returns the entry label of the given function and takes overrides into account.
 	eth::AssemblyItem getVirtualFunctionEntryLabel(FunctionDefinition const& _function) const;
-	/// Returns the distance of the given local variable from the top of the local variable stack.
+	ModifierDefinition const& getFunctionModifier(std::string const& _name) const;
+	/// Returns the distance of the given local variable from the bottom of the stack (of the current function).
 	unsigned getBaseStackOffsetOfVariable(Declaration const& _declaration) const;
 	/// If supplied by a value returned by @ref getBaseStackOffsetOfVariable(variable), returns
 	/// the distance of that variable from the current top of the stack.
@@ -112,14 +115,14 @@ private:
 	u256 m_stateVariablesSize = 0;
 	/// Storage offsets of state variables
 	std::map<Declaration const*, u256> m_stateVariables;
-	/// Offsets of local variables on the stack (relative to stack base).
+	/// Positions of local variables on the stack.
 	std::map<Declaration const*, unsigned> m_localVariables;
-	/// Sum of stack sizes of local variables
-	unsigned m_localVariablesSize;
 	/// Labels pointing to the entry points of funcitons.
 	std::map<Declaration const*, eth::AssemblyItem> m_functionEntryLabels;
 	/// Labels pointing to the entry points of function overrides.
 	std::map<std::string, eth::AssemblyItem> m_virtualFunctionEntryLabels;
+	/// Mapping to obtain function modifiers by name. Should be filled from derived to base.
+	std::map<std::string, ModifierDefinition const*> m_functionModifiers;
 };
 
 }
diff --git a/NameAndTypeResolver.cpp b/NameAndTypeResolver.cpp
index c67cd727..43201fe1 100644
--- a/NameAndTypeResolver.cpp
+++ b/NameAndTypeResolver.cpp
@@ -60,6 +60,11 @@ void NameAndTypeResolver::resolveNamesAndTypes(ContractDefinition& _contract)
 		ReferencesResolver resolver(*structDef, *this, &_contract, nullptr);
 	for (ASTPointer<VariableDeclaration> const& variable: _contract.getStateVariables())
 		ReferencesResolver resolver(*variable, *this, &_contract, nullptr);
+	for (ASTPointer<ModifierDefinition> const& modifier: _contract.getFunctionModifiers())
+	{
+		m_currentScope = &m_scopes[modifier.get()];
+		ReferencesResolver resolver(*modifier, *this, &_contract, nullptr);
+	}
 	for (ASTPointer<FunctionDefinition> const& function: _contract.getDefinedFunctions())
 	{
 		m_currentScope = &m_scopes[function.get()];
@@ -227,6 +232,19 @@ void DeclarationRegistrationHelper::endVisit(FunctionDefinition&)
 	closeCurrentScope();
 }
 
+bool DeclarationRegistrationHelper::visit(ModifierDefinition& _modifier)
+{
+	registerDeclaration(_modifier, true);
+	m_currentFunction = &_modifier;
+	return true;
+}
+
+void DeclarationRegistrationHelper::endVisit(ModifierDefinition&)
+{
+	m_currentFunction = nullptr;
+	closeCurrentScope();
+}
+
 void DeclarationRegistrationHelper::endVisit(VariableDefinition& _variableDefinition)
 {
 	// Register the local variables with the function
@@ -293,8 +311,7 @@ void ReferencesResolver::endVisit(VariableDeclaration& _variable)
 
 bool ReferencesResolver::visit(Return& _return)
 {
-	solAssert(m_returnParameters, "Return parameters not set.");
-	_return.setFunctionReturnParameters(*m_returnParameters);
+	_return.setFunctionReturnParameters(m_returnParameters);
 	return true;
 }
 
diff --git a/NameAndTypeResolver.h b/NameAndTypeResolver.h
index f97c7ae5..ba327a59 100644
--- a/NameAndTypeResolver.h
+++ b/NameAndTypeResolver.h
@@ -100,6 +100,8 @@ private:
 	void endVisit(StructDefinition& _struct);
 	bool visit(FunctionDefinition& _function);
 	void endVisit(FunctionDefinition& _function);
+	bool visit(ModifierDefinition& _modifier);
+	void endVisit(ModifierDefinition& _modifier);
 	void endVisit(VariableDefinition& _variableDefinition);
 	bool visit(VariableDeclaration& _declaration);
 
@@ -109,7 +111,7 @@ private:
 
 	std::map<ASTNode const*, DeclarationContainer>& m_scopes;
 	Declaration const* m_currentScope;
-	FunctionDefinition* m_currentFunction;
+	VariableScope* m_currentFunction;
 };
 
 /**
diff --git a/Parser.cpp b/Parser.cpp
index fcabdb29..baddbad8 100644
--- a/Parser.cpp
+++ b/Parser.cpp
@@ -121,6 +121,7 @@ ASTPointer<ContractDefinition> Parser::parseContractDefinition()
 	vector<ASTPointer<StructDefinition>> structs;
 	vector<ASTPointer<VariableDeclaration>> stateVariables;
 	vector<ASTPointer<FunctionDefinition>> functions;
+	vector<ASTPointer<ModifierDefinition>> modifiers;
 	if (m_scanner->getCurrentToken() == Token::IS)
 		do
 		{
@@ -152,13 +153,15 @@ ASTPointer<ContractDefinition> Parser::parseContractDefinition()
 			stateVariables.push_back(parseVariableDeclaration(allowVar));
 			expectToken(Token::SEMICOLON);
 		}
+		else if (currentToken == Token::MODIFIER)
+			modifiers.push_back(parseModifierDefinition());
 		else
-			BOOST_THROW_EXCEPTION(createParserError("Function, variable or struct declaration expected."));
+			BOOST_THROW_EXCEPTION(createParserError("Function, variable, struct or modifier declaration expected."));
 	}
 	nodeFactory.markEndPosition();
 	expectToken(Token::RBRACE);
 	return nodeFactory.createNode<ContractDefinition>(name, docString, baseContracts, structs,
-													  stateVariables, functions);
+													  stateVariables, functions, modifiers);
 }
 
 ASTPointer<InheritanceSpecifier> Parser::parseInheritanceSpecifier()
@@ -189,10 +192,18 @@ ASTPointer<FunctionDefinition> Parser::parseFunctionDefinition(bool _isPublic, A
 	ASTPointer<ASTString> name(expectIdentifierToken());
 	ASTPointer<ParameterList> parameters(parseParameterList());
 	bool isDeclaredConst = false;
-	if (m_scanner->getCurrentToken() == Token::CONST)
+	vector<ASTPointer<ModifierInvocation>> modifiers;
+	while (true)
 	{
-		isDeclaredConst = true;
-		m_scanner->next();
+		if (m_scanner->getCurrentToken() == Token::CONST)
+		{
+			isDeclaredConst = true;
+			m_scanner->next();
+		}
+		else if (m_scanner->getCurrentToken() == Token::IDENTIFIER)
+			modifiers.push_back(parseModifierInvocation());
+		else
+			break;
 	}
 	ASTPointer<ParameterList> returnParameters;
 	if (m_scanner->getCurrentToken() == Token::RETURNS)
@@ -212,8 +223,8 @@ ASTPointer<FunctionDefinition> Parser::parseFunctionDefinition(bool _isPublic, A
 	nodeFactory.setEndPositionFromNode(block);
 	bool const c_isConstructor = (_contractName && *name == *_contractName);
 	return nodeFactory.createNode<FunctionDefinition>(name, _isPublic, c_isConstructor, docstring,
-													  parameters,
-													  isDeclaredConst, returnParameters, block);
+													  parameters, isDeclaredConst, modifiers,
+													  returnParameters, block);
 }
 
 ASTPointer<StructDefinition> Parser::parseStructDefinition()
@@ -242,6 +253,50 @@ ASTPointer<VariableDeclaration> Parser::parseVariableDeclaration(bool _allowVar)
 	return nodeFactory.createNode<VariableDeclaration>(type, expectIdentifierToken());
 }
 
+ASTPointer<ModifierDefinition> Parser::parseModifierDefinition()
+{
+	ScopeGuard resetModifierFlag([this]() { m_insideModifier = false; });
+	m_insideModifier = true;
+
+	ASTNodeFactory nodeFactory(*this);
+	ASTPointer<ASTString> docstring;
+	if (m_scanner->getCurrentCommentLiteral() != "")
+		docstring = make_shared<ASTString>(m_scanner->getCurrentCommentLiteral());
+
+	expectToken(Token::MODIFIER);
+	ASTPointer<ASTString> name(expectIdentifierToken());
+	ASTPointer<ParameterList> parameters;
+	if (m_scanner->getCurrentToken() == Token::LPAREN)
+		parameters = parseParameterList();
+	else
+	{
+		// create an empty parameter list at a zero-length location
+		ASTNodeFactory nodeFactory(*this);
+		nodeFactory.setLocationEmpty();
+		parameters = nodeFactory.createNode<ParameterList>(vector<ASTPointer<VariableDeclaration>>());
+	}
+	ASTPointer<Block> block = parseBlock();
+	nodeFactory.setEndPositionFromNode(block);
+	return nodeFactory.createNode<ModifierDefinition>(name, docstring, parameters, block);
+}
+
+ASTPointer<ModifierInvocation> Parser::parseModifierInvocation()
+{
+	ASTNodeFactory nodeFactory(*this);
+	ASTPointer<Identifier> name = ASTNodeFactory(*this).createNode<Identifier>(expectIdentifierToken());
+	vector<ASTPointer<Expression>> arguments;
+	if (m_scanner->getCurrentToken() == Token::LPAREN)
+	{
+		m_scanner->next();
+		arguments = parseFunctionCallArguments();
+		nodeFactory.markEndPosition();
+		expectToken(Token::RPAREN);
+	}
+	else
+		nodeFactory.setEndPositionFromNode(name);
+	return nodeFactory.createNode<ModifierInvocation>(name, arguments);
+}
+
 ASTPointer<TypeName> Parser::parseTypeName(bool _allowVar)
 {
 	ASTPointer<TypeName> type;
@@ -354,8 +409,16 @@ ASTPointer<Statement> Parser::parseStatement()
 			nodeFactory.setEndPositionFromNode(expression);
 		}
 		statement = nodeFactory.createNode<Return>(expression);
+		break;
 	}
-	break;
+	case Token::IDENTIFIER:
+		if (m_insideModifier && m_scanner->getCurrentLiteral() == "_")
+		{
+			statement = ASTNodeFactory(*this).createNode<PlaceholderStatement>();
+			m_scanner->next();
+			return statement;
+		}
+	// fall-through
 	default:
 		statement = parseVarDefOrExprStmt();
 	}
diff --git a/Parser.h b/Parser.h
index 5905a042..f85c3186 100644
--- a/Parser.h
+++ b/Parser.h
@@ -53,6 +53,8 @@ private:
 	ASTPointer<FunctionDefinition> parseFunctionDefinition(bool _isPublic, ASTString const* _contractName);
 	ASTPointer<StructDefinition> parseStructDefinition();
 	ASTPointer<VariableDeclaration> parseVariableDeclaration(bool _allowVar);
+	ASTPointer<ModifierDefinition> parseModifierDefinition();
+	ASTPointer<ModifierInvocation> parseModifierInvocation();
 	ASTPointer<TypeName> parseTypeName(bool _allowVar);
 	ASTPointer<Mapping> parseMapping();
 	ASTPointer<ParameterList> parseParameterList(bool _allowEmpty = true);
@@ -90,6 +92,8 @@ private:
 	ParserError createParserError(std::string const& _description) const;
 
 	std::shared_ptr<Scanner> m_scanner;
+	/// Flag that signifies whether '_' is parsed as a PlaceholderStatement or a regular identifier.
+	bool m_insideModifier = false;
 };
 
 }
diff --git a/Token.h b/Token.h
index 552e9a75..55ec8e8e 100644
--- a/Token.h
+++ b/Token.h
@@ -159,6 +159,7 @@ namespace solidity
 	K(IF, "if", 0)                                                     \
 	K(IMPORT, "import", 0)                                             \
 	K(MAPPING, "mapping", 0)                                           \
+	K(MODIFIER, "modifier", 0)                                         \
 	K(NEW, "new", 0)                                                   \
 	K(PUBLIC, "public", 0)                                             \
 	K(PRIVATE, "private", 0)                                           \
diff --git a/Types.cpp b/Types.cpp
index 2446c513..c75cecfa 100644
--- a/Types.cpp
+++ b/Types.cpp
@@ -724,6 +724,38 @@ MemberList const& TypeType::getMembers() const
 	return *m_members;
 }
 
+ModifierType::ModifierType(const ModifierDefinition& _modifier)
+{
+	TypePointers params;
+	params.reserve(_modifier.getParameters().size());
+	for (ASTPointer<VariableDeclaration> const& var: _modifier.getParameters())
+		params.push_back(var->getType());
+	swap(params, m_parameterTypes);
+}
+
+bool ModifierType::operator==(Type const& _other) const
+{
+	if (_other.getCategory() != getCategory())
+		return false;
+	ModifierType const& other = dynamic_cast<ModifierType const&>(_other);
+
+	if (m_parameterTypes.size() != other.m_parameterTypes.size())
+		return false;
+	auto typeCompare = [](TypePointer const& _a, TypePointer const& _b) -> bool { return *_a == *_b; };
+
+	if (!equal(m_parameterTypes.cbegin(), m_parameterTypes.cend(),
+			   other.m_parameterTypes.cbegin(), typeCompare))
+		return false;
+	return true;
+}
+
+string ModifierType::toString() const
+{
+	string name = "modifier (";
+	for (auto it = m_parameterTypes.begin(); it != m_parameterTypes.end(); ++it)
+		name += (*it)->toString() + (it + 1 == m_parameterTypes.end() ? "" : ",");
+	return name + ")";
+}
 
 MagicType::MagicType(MagicType::Kind _kind):
 	m_kind(_kind)
diff --git a/Types.h b/Types.h
index e6c99fe3..935c39eb 100644
--- a/Types.h
+++ b/Types.h
@@ -75,7 +75,7 @@ class Type: private boost::noncopyable, public std::enable_shared_from_this<Type
 public:
 	enum class Category
 	{
-		INTEGER, INTEGER_CONSTANT, BOOL, REAL, STRING, CONTRACT, STRUCT, FUNCTION, MAPPING, VOID, TYPE, MAGIC
+		INTEGER, INTEGER_CONSTANT, BOOL, REAL, STRING, CONTRACT, STRUCT, FUNCTION, MAPPING, VOID, TYPE, MODIFIER, MAGIC
 	};
 
 	///@{
@@ -464,6 +464,27 @@ private:
 
 
 /**
+ * The type of a function modifier. Not used for anything for now.
+ */
+class ModifierType: public Type
+{
+public:
+	virtual Category getCategory() const override { return Category::MODIFIER; }
+	explicit ModifierType(ModifierDefinition const& _modifier);
+
+	virtual TypePointer binaryOperatorResult(Token::Value, TypePointer const&) const override { return TypePointer(); }
+	virtual bool canBeStored() const override { return false; }
+	virtual u256 getStorageSize() const override { BOOST_THROW_EXCEPTION(InternalCompilerError() << errinfo_comment("Storage size of non-storable type type requested.")); }
+	virtual bool canLiveOutsideStorage() const override { return false; }
+	virtual bool operator==(Type const& _other) const override;
+	virtual std::string toString() const override;
+
+private:
+	TypePointers m_parameterTypes;
+};
+
+
+/**
  * Special type for magic variables (block, msg, tx), similar to a struct but without any reference
  * (it always references a global singleton by name).
  */
diff --git a/grammar.txt b/grammar.txt
index 11d99854..b97dac5d 100644
--- a/grammar.txt
+++ b/grammar.txt
@@ -1,14 +1,14 @@
 ContractDefinition = 'contract' Identifier
                      ( 'is' InheritanceSpecifier (',' InheritanceSpecifier )* )?
                      '{' ContractPart* '}'
-ContractPart = VariableDeclaration ';' | StructDefinition |
+ContractPart = VariableDeclaration ';' | StructDefinition | ModifierDefinition |
                FunctionDefinition | 'public:' | 'private:'
 
 InheritanceSpecifier = Identifier ( '(' Expression ( ',' Expression )* ')' )?
 StructDefinition = 'struct' Identifier '{'
                      ( VariableDeclaration (';' VariableDeclaration)* )? '}
-
-FunctionDefinition = 'function' Identifier ParameterList 'const'?
+ModifierDefinition = 'modifier' Identifier ParameterList? Block
+FunctionDefinition = 'function' Identifier ParameterList ( Identifier | 'constant' )*
                      ( 'returns' ParameterList )? Block
 ParameterList = '(' ( VariableDeclaration (',' VariableDeclaration)* )? ')'
 // semantic restriction: mappings and structs (recursively) containing mappings